This proposal focuses on the functional expression of the oncogene sequences of two avian acute leukemia viruses, myelocytomatosis virus (MC29) and avian myeloblastosis virus (AMV). MC29 and AMV are replication defective leukemia viruses capable of inducing acute leukemias of various types as well as carcinomas and sarcomas. Cells transformed by these viruses display a distinct phenotype of differentiation. To investigate the structure and function of the MC29 oncogene (myc) and the AMV oncogene (myb), we will use recombinant DNA technology coupled with DNA modification techniques to alter the DNA sequence of molecular clone of MC29 and AMV. The isolation and characterization of engineered mutants and their reintroduction into cells via transfection procedures will permit us to assess the affects of specific mutations on myc andmyb gene expression. To facilitate the characterization of the myc and myb proteins and their normal cell counterparts, we propose to utilize bacterial expression of the respective myc and myb sequences as a means of obtaining large amounts of viral protein. These proteins will be used to raise antibodies to myc and myb specific determinants. Using the reagents generated above, namely a group of well-defined mutants and antisera to defined protein sequences, we propose to examine the expression of myc and myb gene products in fibroblast ad hematopoietic cells of normal and transformed origin. The understanding of the structure and function of oncogene proteins in essential for our ultimate knowledge of the mechanisms of oncogenesis. The combined genetic and biochemical approach to the definition of the structure and function of the myc and myb oncogenes should provide valuable information regarding this process.